With increasing industrial development, diverse electronic devices are used to achieve various purposes. An electronic device comprises a plurality of electronic components. Generally, different kinds of electronic components are operated by using different voltages.
As known, a power supply is essential for many electronic devices such as personal computers, industrial computers, servers, communication products or network products. Usually, the user may simply plug a power supply into an AC wall outlet commonly found in most homes or offices so as to receive an AC voltage. The power supply will convert the AC voltage into a regulated DC output voltage for powering the electronic device. The regulated DC output voltage is transmitted to the electronic device through a power cable.
Generally, power supply apparatuses are classified into two types, i.e. a linear power supply and a switching power supply (SPS). A linear power supply principally comprises a transformer, a diode rectifier and a capacitor filter. The linear power supply is advantageous due to its simplified circuitry and low fabricating cost. Since the linear power supply has bulky volume, the linear power supply is not applicable to a slim-type electronic device. In addition, the converting efficiency of the linear power supply is too low to comply with the power-saving requirements. In comparison with the linear power supply, the switching power supply has reduced volume but increased converting efficiency. That is, the switching power supply is applicable to the slim-type electronic device and may meet with the power-saving requirements.
The conventional two-stage switching power supply comprises a first-stage power circuit and a second-stage power circuit. By the first-stage power circuit, an input AC voltage is converted into a bus voltage having a constant voltage value. By the second-stage power circuit, the bus voltage is converted into an output voltage having a rated voltage value, which is required for powering an electronic device. Generally, the magnitude of the input AC voltage needs to be maintained within a specified range (e.g. 100˜120V) in order to generate the constant bus voltage by the first-stage power circuit. If the magnitude of the input AC voltage is beyond the specified range, the magnitude of the bus voltage is also beyond the predetermined voltage value. Since the second-stage power circuit is designed to receive the constant bus voltage (e.g. 110V) and generate the output voltage, the magnitude of the output voltage is also altered as the magnitude of the bus voltage is changed.
From the above discussion, the variation amount of the input AC voltage is very tiny in order to allow the first-stage power circuit to generate the constant bus voltage. If the magnitude of the bus voltage is beyond the predetermined voltage value, the output voltage generated by the second-stage power circuit fails to be maintained at the rated voltage value. If the input AC voltage is subject to a sudden variation or interruption (e.g. from a lightning stroke or activation of a motor), the output voltage is also subject to a sudden variation or interruption. Moreover, the conventional two-stage switching power supply has an additional power factor correction (PFC) circuit for achieving a power factor correction purpose. As known, the PFC circuit increases complexity of the whole two-stage switching power supply and increases the fabricating cost.
Therefore, there is a need of providing an improved two-stage switching power supply so as to obviate the drawbacks encountered from the prior art.